Data Communications Networking Devices - 4th Ed.pdf

1.15 THE DATA LINK LAYER ______________________________________________________ 137operate as an IBM 2780 or 3780 workstation or as an IBM 3270 type of interactiveterminal.The bisynchronous transmission protocol can be used in a variety of transmissioncodes on a large number of medium- to high-speed equipment. Some ofthe constraints of this protocol are that it is limited to half-duplex transmission andthat it requires the acknowledgement of the receipt of every block of datatransmitted. A large number of protocols have been developed owing to the successof the BISYNC protocol. Some of these protocols are bit-oriented,whereasBISYNC is a character-oriented protocol,and some permit full-duplex transmission,whereasBISYNC is limited to half-duplex transmission.Data code useMost bisynchronous protocols support several data codes including the 6-bittranscode SBT),7-bit ASCII and 8-bit EBCDIC. Normally,error control isobtained by using a two-dimensional parity check LRC/VRC) when transmissionis in ASCII. When transmission is in EBCDIC the CRC-16 polynomial is used togenerate a block-check character,while the use of the SBT code is accompanied bythe use of a CRC-12 polynomial.Figure 1.77 illustrates the generalized bisynchronous block structure. Forsynchronization,most BISYNC protocols require the transmission and detectionof two successive synchronization SYN) characters. The start of message controlcode is normally the STX communications control character. The end of messagecontrol code can be either the end of text ETX),end of transmission block ETB),or the end of transmission EOT) character; the actual character,however,depends upon whether the block is one of many blocks,the end of the transmissionblock,or the end of the transmission session.Figure 1.77Generalized BSC block structureThe ETX character is used to terminate a block of data started with a SOH orSTX character which was transmitted as an entity. SOH identi®es the beginning ofa block of control information,such as a destination address,priority and messagesequence number. The STX character denotes both the end of the message headerand the beginning of the actual content of the message. A BCC character alwaysfollows an ETX character. Since the ETX only signi®es the end of a message,it